Quick Looks X-ray Mode

You can make icons semi-transparent in Quick Look, with previews of the folder’s contents in the folder.

defaults write com.apple.finder QLEnableXRayFolders 1

Once run, and the Finder is killed (or a reboot performed), Quick Look ends up looking something like this:

To undo it, simply change the 1 to a 0 from earlier:

defaults write com.apple.finder QLEnableXRayFolders 0